Methods and apparatus for vertebral body distraction and fusion employing flexure members

ABSTRACT

Improved methods and apparatuses for vertebral body distraction and fusion in accordance with various embodiments of the present invention employ flexure members. Flexure members connect a plurality of structural members to end plates on one end and blocks on another end. Upon insertion into the disc space, a drive screw or similar mechanism can be actuated to drive expansion blocks closer together, which causes flexure members to deflect, resulting in expansion of the structural members and distraction of the end plates. The distracted device can then remain in the body and be used for vertebral body fusion.

PRIORITY

The present application claims the benefit of U.S. ProvisionalApplication No. 61/142,104, filed Dec. 31, 2008 and U.S. ProvisionalApplication No. 61/291,203, filed Dec. 30, 2009, each of which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to the distraction and fusion of vertebralbodies. More specifically, the present invention relates to devices andmethods for distraction and fusion of vertebral bodies employingflexural members.

BACKGROUND OF THE INVENTION

The concept of intervertebral fusion for the cervical and lumbar spinefollowing a discectomy was generally introduced in the 1960s. Itinvolved coring out a bone graft from the hip and implanting the graftinto the disc space. The disc space was prepared by coring out the spaceto match the implant. The advantages of this concept were that itprovided a large surface area of bone to bone contact and placed thegraft under loading forces that allowed osteoconduction and inductionenhancing bone fusion. However, the technique is seldom practiced todaydue to numerous disadvantages including lengthy operation time,destruction of a large portion of the disc space, high risk of nerveinjury, and hip pain after harvesting the bone graft.

Presently, at least two devices are commonly used to perform theintervertebral portion of an intervertebral body fusion: the first isthe distraction device and the second is the intervertebral body fusiondevice, often referred to as a cage. Cages can be implanted asstandalone devices or as part of a circumferential fusion approach withpedicle screws and rods. The concept is to introduce an implant thatwill distract a collapsed disc and decompress the nerve root to allowload sharing to enhance bone formation, and to implant a device that issmall enough to allow implantation with minimal retraction and pullingon nerves.

In a typical intervertebral body fusion procedure, a portion of theintervertebral disc is first removed from between the vertebral bodies.This can be done through either a direct open approach or a minimallyinvasive approach. Disc shavers, pituitary rongeours, curettes, and/ordisc scrapers can be used to remove the nucleus and a portion of eitherthe anterior or posterior annulus to allow implantation and access tothe inner disc space. The distraction device is inserted into thecleared space to enlarge the disc space and the vertebral bodies areseparated by actuating the distraction device. Enlarging the disc spaceis important because it also opens the foramen where the nerve rootexists. It is important that during the distraction process one does notover-distract the facet joints. An intervertebral fusion device is nextinserted into the distracted space and bone growth factor, such asautograft, a collagen sponge with bone morphogenetic protein, or otherbone enhancing substance may be inserted into the space within theintervertebral fusion device to promote the fusion of the vertebralbodies.

Intervertebral fusion and distraction can be performed through anterior,posterior, oblique, and lateral approaches. Each approach has its ownanatomic challenges, but the general concept is to fuse adjacentvertebra in the cervical thoracic or lumbar spine. Devices have beenmade from various materials. Such materials include cadaveric cancellousbone, carbon fiber, titanium and polyetheretherketone (PEEK). Deviceshave also been made into different shapes such as a bean shape, footballshape, banana shape, wedge shape and a threaded cylindrical cage.

SUMMARY OF THE INVENTION

Improved methods and apparatuses for vertebral body distraction andfusion in accordance with various embodiments of the present inventionemploy flexure members. Flexure members connect a plurality ofstructural members to end plates on one end and blocks on another end.Upon insertion into the disc space, a drive screw or similar mechanismcan be actuated to drive expansion blocks closer together, which causesflexure members to deflect, resulting in expansion of the structuralmembers and distraction of the end plates. The distracted device canthen remain in the body and be used for vertebral body fusion.

In one embodiment, a device can be used for both intervertebral bodydistraction and fusion. The device includes a one-piece device bodycomprised of a ductile material. The device body can include a pair ofopposed end plates, a plurality of structural members, and flexuremembers attaching one end of each structural member to an end plate andthe other end of each structural member to a block. The device body caninclude two sets of structural members, or struts, on each side or threeor more struts. Drive screws, for example, can be inserted throughexpansion blocks and actuated to drive the expansion blocks closertogether, resulting in deflection of the flexure members, which causesexpansion of the struts and distraction of the end plates. The flexuremembers allow a one-piece device to behave similarly to a device havingmultiple parts and rotating pin joints.

In another embodiment, a method of intervertebral body distraction andfusion involves implantation of a distractible intervertebral bodyfusion device. Once the device is inserted into the disc space with animplantation tool, drive screws can be actuated to deflect flexuremembers on device, causing end plates to distract. After the end plateshave reached a desired distraction, a bone growth stimulant can bedelivered into the open area of the distracted device. The implantationtool can be withdrawn, and the device can remain in the body to aid inthe fusion process and support in-vivo loads. In another embodiment, thebone growth stimulant can be added to a chamber within the device priorto implantation of the device.

In one embodiment, the flexure members are arranged so as to create adouble-sided rolling flexure arrangement that enables rolling contactsof the flexure element between two rolling contact surfaces. In oneembodiment, the two rolling contact surfaces are each curved. In anotherembodiment, the rolling contact surface closer to the strut element isstraight, while the other rolling contact surface is convex as viewedfrom the long axis of the strut. In this way, a system having rigidbars, links or struts can form a multiple bar linkage by the use of theflexure members as described in the various embodiments as revolutejoints. Advantages of these arrangements permit increases in theeffective stiffness, strength, and fatigue life of the apparatus and theability to resist buckling, while permitting a large range of motion.

The above summary of the various embodiments of the invention is notintended to describe each illustrated embodiment or every implementationof the invention. This summary represents a simplified overview ofcertain aspects of the invention to facilitate a basic understanding ofthe invention and is not intended to identify key or critical elementsof the invention or delineate the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1A is a perspective view of an embodiment of a distractibleintervertebral body fusion device according to an aspect of the presentinvention.

FIG. 1B is a side view of the distractible intervertebral body fusiondevice of FIG. 1A.

FIG. 1C is an end view of the distractible intervertebral body fusiondevice of FIG. 1A.

FIG. 2A is a perspective view of an embodiment of a distractibleintervertebral body fusion device according to an aspect of the presentinvention.

FIG. 2B is a side view of the distractible intervertebral body fusiondevice of FIG. 2A.

FIG. 3 is a perspective view of an embodiment of a distractibleintervertebral body fusion device and an insertion tool according to anaspect of the present invention.

FIG. 4 is a side view of an embodiment of a distractible intervertebralbody fusion device being inserted into a disc space according to anaspect of the present invention.

FIG. 5 is a perspective view of a pair of distractible intervertebralbody fusion devices inserted into a disc space according to an aspect ofthe present invention.

FIG. 6 is a side view of an embodiment of a distractible intervertebralbody fusion device according to an aspect of the present invention.

FIG. 7 is a side view of an embodiment of a distractible intervertebralbody fusion device according to an aspect of the present invention.

FIG. 8A is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8B is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8C is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8D is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8E is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8F is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 8G is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 9 is a side view of an embodiment of a distractible intervertebralbody fusion device according to an aspect of the present invention.

FIG. 10 is a perspective view of the distractible intervertebral bodyfusion device of FIG. 9.

FIG. 11A is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 11B is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 11C is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 12 is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 13A is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 13B is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 13C is a partial view of a portion of an embodiment of adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 14 is a side view of an embodiment of a distractible intervertebralbody fusion device according to an aspect of the present invention.

FIG. 15A is a simplified side view of an embodiment of a distractibleintervertebral body fusion device according to an aspect of the presentinvention.

FIG. 15B is a simplified side view of an embodiment of a distractibleintervertebral body fusion device according to an aspect of the presentinvention.

FIG. 16 is a side view of a circular flexure.

FIG. 17 is a side view of an elliptical flexure.

FIG. 18 is a side view of a leaf flexure.

FIG. 19A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 19B is a side view of the distractible intervertebral body fusiondevice of FIG. 19A.

FIG. 20A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 20B is a side view of the distractible intervertebral body fusiondevice of FIG. 20A.

FIG. 21A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 21B is a side view of the distractible intervertebral body fusiondevice of FIG. 21A.

FIG. 22A is a perspetive view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 22B is a side view of the distractible intervertebral body fusiondevice of FIG. 22A.

FIG. 23A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 23B is a side view of the distractible intervertebral body fusiondevice of FIG. 23A.

FIG. 24 is an end view of a distractible intervertebral body fusiondevice according to an aspect of the present invention.

FIG. 25 is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 26A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 26B is a side view of the distractible intervertebral body fusiondevice of FIG. 26A.

FIG. 27A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 27B is a side view of the distractible intervertebral body fusiondevice of FIG. 27A.

FIG. 27C is a simplified side view of the distractible intervertebralbody fusion device of FIG. 27A.

FIG. 27D is a simplified side view of the distractible intervertebralbody fusion device of FIG. 27A.

FIG. 28A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 28B is a side view of the distractible intervertebral body fusiondevice of FIG. 28A.

FIG. 29A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 29B is a side view of the distractible intervertebral body fusiondevice of FIG. 29A.

FIG. 30A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 30B is a side view of the distractible intervertebral body fusiondevice of FIG. 30A.

FIG. 30C is a simplified side view of the distractible intervertebralbody fusion device of FIG. 30A.

FIG. 30D is a simplified side view of the distractible intervertebralbody fusion device of FIG. 30A.

FIG. 31A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 31B is an end view of the distractible intervertebral body fusiondevice of FIG. 31A.

FIG. 31C is a side view of the distractible intervertebral body fusiondevice of FIG. 31A.

FIG. 32A is a perspective view of a distractible intervertebral bodyfusion device according to an aspect of the present invention.

FIG. 32B is an end view of the distractible intervertebral body fusiondevice of FIG. 32A.

FIG. 33A is a partial perspective view of an embodiment of an insertiontool according to an aspect of the present invention.

FIG. 33B is a partial top view of the insertion tool of FIG. 33A and adistractible intervertebral body fusion device according to an aspect ofthe present invention.

FIG. 33C is a partial perspective view of the insertion tool of FIG.33A.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, one skilled in the artwill recognize that the present invention may be practiced without thesespecific details. In other instances, well-known methods, procedures,and components have not been described in detail so as to notunnecessarily obscure aspects of the various embodiments of the presentinvention.

Referring to FIGS. 1A-1C and 2A-2B there can be seen a distractibleintervertebral body fusion device 100 according to an aspect of thepresent invention. Device 100 includes a device body 102. Device body102 can include a nose portion 104, a rear portion 106, a pair ofopposed end plates 108, structural members 110 and flexure members 112attaching one end of the structural members 110 to end plates 108 andthe other end of structural members 110 to blocks 114 a, 114 b.

Device body 102 can include two sets of structural members 110, orstruts, on each side (FIGS. 1A-1D) or can include three, or more, setsof structural members 110 on each side (FIGS. 2A-2B). As will bediscussed in more detail herein, addition of a third strut providesgreater stability to the device 100. Flexure members 112 are thin stripsof material that connect the structural members to the end plates 108and expansion blocks 114. The flexure members 112 allow a one-piecedevice 100 to behave similarly to a device having multiple parts and arotating pin joint. Flexure members 112 can, for example, be bandflexures (FIGS. 1A-1C and 2A-2B), circular flexures (FIG. 16),elliptical flexures (FIGS. 17 and 20A-B), or leaf flexures (FIGS. 18,19A-B, 21A-B and 22A-B).

In one embodiment, each end plate 108 includes a rectangular opening116. Opening can be used to facilitate bone growth through the device100. In other embodiments, opening 116 can be filled with a gel, rubber,or other complaint material that can replicate the nucleus of aninterverterbral disc and supplement the strength of the flexures 112 incompressive, shear, and torsional loading conditions. Alternatively, agenerally solid surface or a surface with multiple openings can beprovided on each end plate 108. End plates 108 can have a rough surfaceor teeth to create friction with the end plates of the vertebra toprevent accidental extrusion of the device 100. In one embodiment, thedevice body 102, or portions of the device body 102, can be overmoldedwith a polymer or other material to supplement the strength of thedevice. For example, long carbon nanotube chains can be applied to thesurface of the device so that as the device distracts the carbonnanotubes align along the surface of the flexures to add to thestability of the device.

Nose portion 104 can be tapered to facilitate the insertion of thedevice 100 into the disc space. Rear portion 106 can also be tapered. Inone embodiment, nose portion 104 and rear portion 106 can be left opento accommodate a tapered delivery shaft that can extend all the waythrough the device 100.

Drive screws 118 can be inserted through guide apertures 120 in rearportion 106 and through expansion blocks 114. Actuation of drive screws118 drives blocks 114 closer together, which causes deflection of theflexure members 112, resulting in expansion of the structural members110 and distraction of the end plates 108. In one embodiment, blocks 114b in FIGS. 1A-1C can be tapped to accommodate drive screws 118 andblocks 114 a can provide a clearance fit with screws 118. When drivescrews 118 are actuated, this allows blocks 114 a to be pulled towardsblocks 114 b, causing the device 100 to distract. Similarly, blocks 114a and 114 c in FIGS. 2A-2B can be tapped and blocks 114 b can provide aclearance fit. In such a configuration, the opposite end from the hex ofscrews 118 can have a shoulder to draw block 114 b towards blocks 114 cand 114 a. In some embodiments, mechanisms other than drive screws canbe used to distract device. Such mechanisms include, for example, apop-rivet mechanism, a sardine key and ribbon, a tourniquet and wire, asaw blade/ratchet, and shape changing materials such as a shape memoryalloy or a conducting polymer actuator. In one embodiment depicted inFIGS. 23A and 23B, a zip-tie-like drive mechanism 819 can be used todistract end plates 808 of device 800. The rear block 814 can include aprojection 821 for engaging the teeth 823 of the drive mechanism 819. Inone embodiment, piezo-electric inch-worm motors can be used to actuatethe movement of blocks 114. In another embodiment, a balloon can beinserted into device and inflated to expand the device. The balloon canremain in the device and function like the nucleus of a disc.

In various embodiments, device body 102 is shaped to be ergonomic.Device body 102 can have various shapes, such as, for example,rectangular, kidney, or football shaped. A kidney or football shapeddevice body 102 maximizes contact between the device and the vertebralbodies because the end plates of vertebrae tend to be slightly concave.One or both ends of the device may also be tapered in order tofacilitate insertion. This minimizes the amount of force needed toinitially insert the device and separate the vertebral bodies. Inaddition, the device may be convex along both its length and its width,or bi-convex. Device 100 can be constructed in various sizes dependingon the type of vertebra and size of patient with which it is being used.

Device body 102 can also be comprised of various materials. In oneembodiment, device is comprised of a ductile material. Such materialscan include, for example, titanium, nitinol, and thermoplastics. In someembodiments, the material near the ends of the flexures 112 can becold-worked to increase the stiffness of the device as it distracts.Heat treating could also be used to alleviate machining stresses andcould be followed by hardening treatment to make the device stiffer.Additionally, in some embodiments the flexures can be affixed to thedevice in subsequent manufacturing steps in order to permit the flexuresto be made from a different material or materials, or materials treateddifferently, than the structural members and end plates of the device.Flexures could also be laminated beams having a core of another stiffmaterial, a soft material such as a foam, or an open core. Having a softor open core would allow the flexures to effectively decrease inthickness as they are bent around the curved surfaces of the struts.This would decrease the amount of strain present in the flexure due tobending, allowing the device to accommodate greater functional loading.

Device 100 can be placed between adjacent vertebra or vertebral bodiesand used both to distract the endplates of the adjacent vertebral bodiesand serve as a fusion device. An insertion tool 200 can be used toinsert a device between vertebral bodies 124 as shown in FIGS. 3-5. Inone embodiment, insertion tool 200 can include a pair of parallelscrewdrivers or wrenches 202 temporarily affixed to the drive screws 118with retainers 204. In one embodiment shown in FIG. 3, insertion tool200 extends rearwardly from device 100. In another embodiment, insertiontool 200 may also extend distally from device 100. In such anembodiment, device 100 can include an open nose portion 104 and rearportion 106 to allow it to be threaded onto insertion tool 200 andinsertion tool 200 can also be used to initially distract the vertebralbodies. Optionally, the insertion tool 200 can include a single handle201 and a gear system 203 where the handle 201 has an internal gearthat, when turned, turns external gears on the shafts that turn thescrews on the device 100 as depicted in FIGS. 33A-C.

Device 100 can be inserted with tapered nose portion 104 first. In oneembodiment, a working channel of 8-26 mm is required for insertion ofthe device. One device 100 can be inserted, or, for additional support,two devices 100 can be inserted as shown in FIG. 5. Two devices 100 canbe especially useful for treating larger patients in which the devicemay encounter higher loads. In another embodiment, three or more smalldevices can be inserted into the disc space in order to very accuratelycontrol the orientation and distance between discs. Three or moredistraction mechanisms may be positioned circumferentially between twocircular endplates to result in very accurate control and orientation ofthe end plates. Such a device would resemble a hexapod. In anotherembodiment, two or more devices may be mated or assembled in the discspace to work congruently in performing distraction either in height orwidth.

Once inserted in the disc space, insertion tool 200 can be actuated torotate drive screws 118. Drive screws 118 can be actuated from the rearof device 106 to allow insertion tool to reposition or, if necessary,remove device 100 prior to disengaging from device 100. Drive screws 118can be actuated the same amount for uniform distraction on both sides ofan embodiment with two drive screws or may be actuated different amountsfor non-uniform distraction with one side of the device 100 higher thanthe other. Non-uniform distraction causes torsional forces on flexures.FIG. 24 depicts a device 100 have non-uniform distraction.Alternatively, an embodiment can be driven with a single flexure andsingle drive screw or with multiple flexures multiplexed to a singledrive screw arrangement.

Unlike many common scissor jacks, such as, for example, car jacks,device 100 can easily be distracted from its lowest, or most compressed,state. This is because the flexure members 112 on each end of a givenstructural member are oriented such that the tensile loads on theflexures do not act towards each other, but instead pass by each other,like passing cars (see arrow A and arrow B in FIG. 1B). Common jacks,which do not utilize flexure members, may have difficulty distractingfrom the lowest state because the tensile loads can act “heads on” witheach other, putting the device under strong internal horizontalcompression but without a significant force component in the verticaldirection at the lowest state that can easily initiate distraction. Thetension in the flexure member required to support a compressive load isequal to the compressive load multiplied by the cosine of the angle ofthe rigid link divided by the sine of the rigid link. Because the sineof zero degrees, the angular position of normal scissor jacks in thecompressed state, is equal to zero, the force required for initialdistraction can be effectively very large. The rigid links of the deviceof various embodiments of the present invention may start off in theposition of zero angular position, but because the flexure members areon opposing sides of the rigid links the effective angular position isnon-zero, making the force required for initial distraction finite andgenerally smaller than a conventional scissor jack.

As drive screws 118 are actuated, the device 100 is distracted as shownin FIGS. 6 and 7. Drive screws 118 (not shown in FIGS. 6 and 7) driveexpansion blocks 114 together, which cause flexure members 112 todeflect thereby expanding structural members 110 to distract end plates108. Referring now to FIGS. 8A-8D, FIGS. 8A and 8D depict a flexuremember 112 and structural member 110 before distraction, whereas FIGS.8B and 8C depict after distraction. Each flexure member 112 beginswrapped around the curved end of the structural member 110. Note in FIG.8A that the flexure 112 rests on the structural member 110. This allowsthe device 100 to carry a large compressive load in the compressed statewithout greatly deforming the flexure 112. As the structural members 110are distracted, the flexure members 112 bend towards flat. In thisembodiment, the flexure members 112 do not bend all the way flat,however, even at maximum distraction of the end plates 108, because theycontact curved backstop 122. This allows the device 100 to carry a largecompressive load in the distracted state without further deforming theflexure 112. Curved backstop 122 has a “frowning eyebrows” configurationin order to provide opposed curved surfaces for opposing flexure members110. Because the flexure members 112 do not have to bend until they arecompletely flat to reach complete distraction, the amount of strain onthe flexure members 112 necessary for complete distraction is minimized.The likelihood of device failure is therefore reduced.

FIGS. 8E-8G depict the behavior of flexures as the device is distracted.Flexure member 112 defines a first open area, or kerf 140 a, betweencurved backstop 122 and flexure member 112 and a second kerf 140 bbetween inner perimeter 142 of structural member 110 and flexure member112. When device 100 is in a collapsed configuration (FIG. 8E), kerf 140a is wider than kerf 140 b. As device distracts, flexure member 112flattens out towards curved backstop 122, so kerf 140 b widens as kerf140 a narrows. The fulcrum around which flexure member 112 bends isshown by arrows 144 a and 144 b. As can be seen in FIGS. 8E-8G, thefulcrum 144 a, 144 b translates along the flexure member 112 as itbends. Fulcrum 144 a, 144 b therefore travels in both vertical andhorizontal directions. This provides for increased distraction of thedevice. As the fulcrum 144 a, 144 b moves along the flexure member 112as the device distracts, a greater portion of the compressive load onthe device 100 is supported by the structural member 110 and,accordingly, the tensile forces on the flexure member 112 are reduced.The device 100 of this embodiment is therefore strongest when it isfully distracted.

Referring now to FIGS. 9 and 10, another embodiment of a distractibleintervertebral body fusion device 300 is shown. Device 300 includes adevice body 302 having a nose portion 304, a rear portion 306, a pair ofopposed end plates 308, structural members 310, flexure members 312, anddrive blocks 314. In some embodiments, as shown in FIGS. 9 and 10, noseportion 304 and rear portion 306 can be open. As described above, nose304 and rear 306 portions can be used to accommodate an insertion toolfor delivery of device 300.

In the embodiment shown in FIGS. 9 and 10, drive blocks 314 and endplates 308 provide outwardly curved backstops 322 for flexure members312 (in contrast to the inwardly curved backstops 122 depicted in theprevious Figures). Flexures 312 curve around backstops 322 as the device300 distracts as depicted in FIGS. 11A-11C. In the collapsed state shownin FIG. 11A, flexure member 312 is parallel to an inner surface 342 ofstructural member 310. As the device 300 distracts, flexure member 312bends around backstop 322, widening kerf 340 b and narrowing kerf 340 a.As shown by arrows 344 a, 344 b, the fulcrum translates along the lengthof flexure member 312 (in both the horizontal and vertical directions)as the device distracts. Fulcrum 344 a, 344 b is always perpendicular toinner surface 342 of structural member 310. This results in the entireload on the device 300 being carried in compression by structuralmembers 310. Therefore, there is little or no tensile force on flexuremembers 312. This allows flexure members 312 to be of a thickness or amaterial such that they enjoy an essentially infinite fatigue life. Thisembodiment allows device to be constructed from a material, such asnitinol, that provides strong compressive support when it is of largedimensions but that distorts easily when slender members of the samematerial are under tension or bending. FIG. 12 depicts a further flexureembodiment employing this principal. The flexure 312 in FIG. 12 is cutan additional length into end plate 308. This can help reduce the stressin the device and may improve fatigue life.

The thickness of the flexure 312 in relation to the bend radius of thecurved backstop 322 determines the fatigue life of the flexure. In someembodiments, flexures can be configured and designed to have very longfatigue life. In one embodiment, a device made from nitinol having athickness of the flexure members 312 that is preferably between 8% and10% of the bend radius of the backstop 322, with a maximum thickness of18% has an infinite fatigue life. In another embodiment, a flexure madefrom PEEK preferably has a thickness that is 4.5% to 6.4% of the bendradius, with a maximum thickness of 15%. In a further embodiment, aflexure comprised of annealed titanium can have a thickness of up to 18%of the bend radius. In other embodiments, flexures can be configured anddesigned to have a finite fatigue life associated with a predeterminedrange of maximum number of cycles of expansion and contraction.

FIGS. 13A-13C depict a partial view of a distractible intervertebralbody fusion device 400 including a further flexure embodiment. Backstop422 on end plate 408 is flat. Flexure 412 begins curved around innersurface 442 of structural member 410 and flattens out, thereby wideningkerf 440 b and narrowing kerf 440 a, as the device distracts. Fulcrum444 a, 444 b again translates along flexure member 412 as the devicedistracts, providing increased distraction. As the device distracts,structural member 410 supports more of the load on device 400 incompression and less is supported by the flexure member 412 in tension.

In some embodiments, following distraction of the device, a bone growthstimulant, such as autograft, bone morphogenic protein, or boneenhancing material, may be delivered into device. In one embodiment,bone growth stimulant is delivered through a hollow chamber in insertiontool before insertion tool is disengaged from device. The devicesupports in-vivo loads during the time fusion occurs between thevertebral bodies and can support axial loads up to four times the weightof the patient. In one embodiment, openings in end plates allow for bonegrowth through the device.

As seen in FIGS. 6 and 7, some embodiments of the device can bedistracted in only one direction, such as vertically. In otherembodiments, the device can be distracted in two directions, such asboth vertically and horizontally. In one embodiment depicted in FIG. 25,the device 600 can be distracted in both the vertical and horizontaldirections. Device 600 includes a plurality of structural members 610and flexures 612 on all four sides of the device 600. Separate drivescrews 618 can be used to control horizontal and vertical distraction.In one embodiment, all drive screws can be controlled by a single drivemember. This would provide for simultaneous horizontal and verticaldistraction. In another embodiment as shown, each drive screw can beindividually controlled in order to allow horizontal and verticaldistraction to be performed independently. This device can be insertedthrough very small openings, which can then be made wider before beingdistracted taller. It is in this configuration that the device retainsits compressive strength during and after vertical compression whilebeing able to be distracted in the horizontal direction. Optionally, thescrews that actuate horizontal expansion may be timed and driventogether and the screws that actuate vertical expansion may be timed anddriven together.

As noted above, a third strut may provide greater stability to certainembodiments of the device over a two-strut design. Referring to FIG. 14,excessive and/or uneven forces on a two-strut design can sometimes causethe device 100 to sublux as shown in FIG. 14. Subluxation causes the endplates 108, structural members 110 and, if during implantation, thedrive screws 118, to become misaligned. This can cause collapse of thedisc space and risks deformity. A third set of structural members may beadded to provide stability to help support excessive and/or uneven loadsand prevent subluxation. In addition, a third strut allows a physician,in some embodiments, the flexibility to control the parallelism of theend plates. As seen in FIG. 15A, the third strut 134 can be positionedapart from the first 132 and second 130 struts in order to maintain theend plates 108 completely parallel. However, in order to establishsagittal alignment, a physician may desire to maintain the end plates108 in a non-parallel position. As can be seen in FIG. 15B, this can beaccomplished by positioning the third strut 134 nearer to or fartherfrom the other struts 130, 132 or by actuating the separate screw drivesat separate rates. In this manner, a physician can configure the device100 to maintain the end plates 108 in a non-parallel position to matchthe curvature of the spine. In one embodiment, the non-parallel positioncan be configured while the device is being implanted by using a drivemechanism that has the flexibility to adjust the position of the thirdstrut 134 with respect to the second strut 132. The length of strut 134may be different from that of strut 132 resulting in the end plates 108being parallel during implantation but growing increasingly lessparallel as the device is distracted.

FIGS. 19A-19B depict a distractible intervertebral body fusion device500 utilizing leaf flexures 512. This device includes small fillets 513where the flexures 512 connect with the structural members 510. In thisembodiment, no portion of the flexures 512 rests on the device body, sothe entirety of any load on the device will be carried by the flexures.FIGS. 21A-21B and 22A-22B also depict devices 500 utilizing leafflexures 512. In these embodiments, there are no fillets at theconnection between the flexures 512 and the structural members 510.FIGS. 20A-20B depict a distractible intervertebral body fusion device700 that utilizes elliptical flexures 712.

FIGS. 27A-27D depict another embodiment of a distractible intervertebralbody fusion device 1000 according to an aspect of the present invention.Device 1000 includes three sets of structural members 1010 on each sideof the device 1000 and utilizes flexures 1012 similar to those depictedin FIGS. 8A-8G. The use of three sets of struts provides greaterstrength and helps avoid buckling or collapse of the device 1000. FIGS.27C and 27D depicted a simplified view of the distracted device 1000under a compressive load. The flexures 1012 in the middle of the device1000 deform differently than the ones on each end due to the asymmetryof the device. The end plates 1008 of this embodiment are depicted asbending slightly under the compressive load. This is because thethickness of the end plates can be selected such that they are able tobend in-vivo to evenly distribute the supportive load of the device overthe endplates of the vertebral bodies.

FIGS. 28A and 28B depict a variation of the device 1000 of FIGS. 27A-27Dhaving a differential screw drive 1018. This allows the flexures 1012 oneach side of the device to be driven at different rates, so that one cancontrol the angle of the device's end plates 1008 once the device 1000is distracted. FIGS. 29A and 29B depict a further variation of thedevice 1000 that includes a wedge 1025 on each end for driving theblocks 1014 together to distract the device 1000. The wedges 1025provide a greater level of compressive strength to the device 100 onceit is distracted than the flexures 1012 do alone. The wedges 1025 alsoreduce the potential for the device to sublux. The wedges 1025 may beshaped or sized such that the device is primarily supported by theflexures and has the ability to sublux slightly but not fully.

Another embodiment of a distractible intervertebral body fusion device1100 according to an aspect of the present invention is depicted inFIGS. 30A-30D. This embodiment uses flexures 1112 similar to those shownin FIGS. 11A-11C. As with device 1000, the middle flexures 1112 ofdevice 1100 deform differently than the ones on each end due to theasymmetric sets of structural members 1110. In this embodiment, the endplates 1108 are thicker and do not deform under the compressive load.

Another embodiment of a distractible intervertebral body fusion device1200 according to an aspect of the present invention is depicted inFIGS. 31A-31C. The device 1200 includes two sets of structural members1210 and flexures 1212 on each side of the device 1200. The device 1200includes four distractible pins 1207 extending between the end plates1208 that resist torsional forces on the device 1200. The pins 1207 alsolimit the device to movement in the vertical direction and eliminate thepossibility of subluxation. A pair of drive screws 1218 can be used todistract the devices. FIGS. 32A and 32B depict a variation of the device1200 that utilizes a single drive screw 1218. FIGS. 32A and 32B alsodepict an embodiment where the blocks 1214 and the backstops 1222 thatcreate the “frowning eyebrows” have been added as separate parts inorder to dramatically thin the kerf that is present in earlierembodiments. The thinning of the kerf will reduce local stresses andstrains in the flexures and increase the fatigue life.

In various embodiments, distractible intervertebral body fusion devicehas a one-piece device body that can be manufactured in a distracted orpartially distracted state. This provides great cost savings overdevices that require multiple pieces to be separately manufactured andassembled. Manufacturing in the distracted state provides additionalclearance for assembly and for access by manufacturing tools, the sizeof which is inversely proportional to the cost of manufacturing. Inaddition, when the device is manufactured in the distracted state, thedevice can be compressed into a position of minimal height whilecompressive stress remains in the flexure members. This compressivestress results in a negative mean stress, which can extend the fatiguelife of the device. In one embodiment, the device can be manufacturedusing wire or sink edm. In another embodiment, the device can bemanufactured using three-dimensional printing techniques or the like. Insome embodiments, portions of the flexures can be machined separatelyand welded to the device. This allows for flexures that have zero kerfand rest completely against the backstops once distracted.

In one embodiment, the surface of the device can be treated to minimizesurface roughness or to reduce pitting of the material within the body.A rough surface or pits can increase the stress on the device, which canresult in shortening of the fatigue life and/or reduce fatigue strength.In one embodiment, the surface can be treated with electro-polishing. Inanother embodiment, the surface can be left untreated because a roughsurface on the end plates helps prevent accidental extrusion of thedevice. In one embodiment, the device can also be coated with a highlyelastic, impermeable material to extend its fatigue life. Specifically,the impermeable material would prevent the corrosive properties of bloodfrom degrading the device. In another embodiment, the device can becomprised of a biocompatible material, so that no coating is necessary.In a further embodiment, the device can be made of a biodegradablematerial designed to degrade in the body at a selected stage of thehealing process, such as after bone fusion.

Numerous other types of supports may be used with the device. Supportscan be used to supplement the compressive strength, bending, ortorsional strength of device. In one embodiment, one or more rigidsupports can be inserted into the open space between end plates afterdistraction to help keep the end plates in their distracted state. Inanother embodiment, chocks can be placed at the intersection ofstructural members in each strut to provide further support for struts.In a further embodiment, a rod and screws can be used with the device aspart of an assembly affixed to the vertebral body.

In another embodiment distractible intervertebral body fusion device900, shown in FIGS. 26A-26B, can comprise a rigid cage capable oftilting front to back and/or side to side. Flexures 912 and/or springscan be oriented around the periphery of the device to allow for tiltingin a variety of axes. A device capable of tilting can be beneficial inthat providing additional degrees of flexibility built into the devicecan promote bone growth, distribute stress across the surface of the endplates, and allow the device to adjust to the curvature of anindividual's spine.

In a further embodiment, the struts comprising structural members,flexures, and blocks can be replaced with large flexures extendingbetween the end plates. Such a device can be non-distractible and can beprovided in different sizes for insertion into variously sized discspaces.

A device in accordance with the various embodiments can be used for avariety of intervertebral fusion applications, including, for example,cervical, thoracic anterior lumbar, trans-foraminal lumbar, extremelateral lumbar, and posterior lumbar. In one embodiment, device can beinserted at 6 mm height and distracted to 14 mm for cervicalapplications and can be inserted at 7 mm and distract to 16 mm for otherapplications. Prototypes of this device have successfully demonstrateddistraction to 220% of the original height. Scissorjacks of the priorart designed for distraction of vertebral bodies are capable ofdistracting to only less than 200% of the original height.

Various embodiments of implantation procedures for these applicationsmay be as follows:

Cervical: The device is implanted via an anterior approach at the C3 toC7 levels using autograft. The device is used with supplemental anteriorplate fixation.

Trans-foraminal lumbar: The device is implanted via a posterior approachfrom the L2 to S1 levels using autograft. The device is used withsupplemental posterior rod fixation.

Posterior lumbar: The device is implanted via a posterior approach fromthe L2 to S1 levels using autograft. Two devices are implanted; one onthe left side of the disc space and the other on the right side of thedisc space. The device is used with supplemental posterior rod fixation.

Anterior lumbar: The device is implanted via an anterior approach fromthe L3 to S1 levels using autograft. The device is used withsupplemental anterior plating fixation of posterior rod fixation.

Extreme lateral lumbar: The device is implanted via a lateral approachfrom the T12 to L4 levels using autograft. The device is used withsupplemental posterior rod fixation.

In another embodiment, the device can be used in vertebral bodyreplacement. After resection of a vertebral body or multiple vertebraedue to fracture or tumor, the device can be distracted to bridge twoseparate vertebrae. The distracted device bridges and supports the voidleft after resection. The device can be constructed in different sizesto accommodate the size difference of cervical, thoracic and lumbarvertebrae.

In another embodiment, the device can be used as an interspinousdistraction device. The device can be placed between two adjacentspinous processes through a minimal access system. The device can beinserted in a collapsed configuration to allow ease of placement. Oncein position, the device can be actuated to lock the vertebrae in adistracted position. The device can have gripping teeth at the point ofcontact with the spinous processes to help fix it in place.

In another embodiment, device can be used for interspinous fusion. Thedevice can be placed between two adjacent spinous processes through aminimal access system in a collapsed configuration. Once in position,the device can be actuated to lock the vertebra in a distractedposition. The device can have a bolt locking mechanism to lock thedevice in the distracted position and to lock the locking plates throughthe spinous processes. The device can also have gripping teeth on theoutside to help keep it in place. Autograft or bone fusion enhancingmaterial can be placed in the open space in device.

In another embodiment, device can be used for intervertebral discreplacement. The device can be placed in a disc space after removal ofthe nucleus pulposus. The device can then be distracted to the properdisc space height for the type of vertebra—cervical, thoracic, orlumbar. The device then functions as a mechanical annulus fibrosis. Thedevice can be used on its own or in combination with a nucleus pulposusimplant or soft posterior rodding system. A PEEK or biogel nucleuspulposus implant can be placed into the open area in the device after itis distracted. The implant and device will function as a mechanical discdevice. The device can be constructed of a flexible material havingsimilar properties to that of a human disc.

In another embodiment, the device can be used as a distractible cage forosteoporotic bone. The device can be constructed of a material with amodulus similar to that of bone and can be coated with a hydroxyappetiteto enhance bone formation in the patient.

In another embodiment, the device can be used in flexure member facetjoint replacement. After resection of a hypertrophic facet joint, thedevice can be actuated and subluxed. Each subluxed plate can be fixed toadjacent vertebrae with a pedicle screw. This will allow motion similarto that of a facet joint and prevent instability. The device can be partof a soft fusion device system and can be used in combination with anintervertebral disc replacement device.

In another embodiment, the device can be used as a programmabledistraction cage with a dynameter and bone stimulator. A programmablemicro-machine actuator device can be implanted within the device. Thedevice is distracted during implantation and can provide force readingsthrough a radio frequency communicator post-surgery. The shape of thedevice can be altered while it is implanted by distracting the endplates with the actuator device, which can result in lordosis, kyphosis,further distraction, or less distraction. In one embodiment, a batterydevice powers the system and can also form a magnetic field that worksas a bone stimulator. The battery life may be limited to a short periodof time, such as one week. Small movements of the device can be used togenerate electrical energy with piezo-electrics or conducting polymersthat may be used to recharge the batteries, capacitors, or other suchpower storage devices. Alternatively, the device may be powered throughan RF inductive or capacitatively coupled arrangement.

In another embodiment, the device can be a self-actuating distractiblecage. The device can be inserted into the disc space in a collapsedstate. Once the device is released, it can slowly distract to a presetheight. In this embodiment, the distraction may be driven by springaction of the flexures.

In another embodiment, the device can be used in facial maxillarysurgery as a fracture lengthening device for mandibular fractures. Thedevice can be designed with narrow end plates having perpendicularplates with holes that allow fixation of each plate to either a proximalor distal fracture. The device can be actuated through a slow springaction flexure mechanism to a preset height. This will allow lengtheningof the defect in cases of fracture bone loss, dysplasia, or hypoplasia.

In another embodiment, device can be used in orthopedic applications asa lengthening nail for distraction of long bone fractures. After anorthopedic fracture occurs with bone loss, a distractible elongatingnail can be placed to lengthen the bone. The elongation occurs over afew days with micrometer movements. This application will involve adistraction device inserted in between the moving portion of the nailsexerting counter-distraction forces, which will provide lengthening ofthe bone.

In another embodiment, device can be used in a gastric band application.Present gastric bands have an inner tube rubber diaphragm that isconstricted via tubing attached to a small reservoir placedsuperficially under the skin in an accessible area. The constrictionmechanism requires an injection of saline into the reservoir by asurgeon a few times a year. A flexure embodiment will include anelliptical device having two flexure members that constrict the centerby opposing distraction forces. The device will be open on one end toallow placement around the upper portion of the stomach. The device caninclude a programmable micro-machine to actuate the flexure members. Thedevice can also measure stomach fundus pressures and diurnal variationsin the size of the stomach.

In another embodiment, the flexure device can be used to replacephalangeal joints in the hand, metatarsal joints in the foot, orcalcaneal-talus joints. These joints can have flexural members implantsthat will allow motion of adjacent bones and limit hyper-extension orhyper-flexion.

In another embodiment, the device can be used to create prostheticlimbs. Specifically, the flexural member can lengthen to adjust for agrowing limb or to make slight adjustment in order to match the size ofa homologous limb.

Various embodiments of systems, devices and methods have been describedherein. These embodiments are given only by way of example and are notintended to limit the scope of the present invention. It should beappreciated, moreover, that the various features of the embodiments thathave been described may be combined in various ways to produce numerousadditional embodiments. Moreover, while various materials, dimensions,shapes, implantation locations, etc. have been described for use withdisclosed embodiments, others besides those disclosed may be utilizedwithout exceeding the scope of the invention.

1. A distractible intervertebral body fusion device for implantationinto an intervertebral disc space in a patient's body, comprising: a topend plate having a top bearing surface configured to interface with anend plate of a superior vertebra of the intervertebral disc space; abottom end plate having a bottom bearing surface configured to interfacewith an end plate of an inferior vertebra of the intervertebral discspace; and at least two struts connecting the top end plate and thebottom end plate, each strut comprising first and second structuralmembers and a block, wherein each structural member is connected at oneend to one of the end plates with a flexure member and at an opposed endto the block with a flexure member, and wherein the blocks includeapertures enabling operable access to an expansion tool such thatactuation of the expansion tool in a first direction drives the blockscloser together, causing the struts to expand, thereby distracting thetop end plate and the bottom end plate with respect to each other. 2.The device of claim 1, wherein the top end plate, bottom end plate andat least two struts comprise a one piece unitary body.
 3. The device ofclaim 1, wherein there are at least two struts on a first side of thedevice and at least two struts on an opposing side of the device.
 4. Thedevice of claim 3, wherein there are three struts on the first side andthree struts on the opposing side.
 5. The device of claim 1, wherein theflexure members are selected from the group consisting of: bandflexures, circular flexures, elliptical flexures, and leaf flexures. 6.The device of claim 1, wherein one of the end plates provides a backstopfor each flexure connecting one of the structural members with one ofthe end plates, the backstop configured to provide a rolling contactwith the flexure and guide a curvature of the flexure as the top andbottom end plates distract.
 7. The device of claim 1, wherein thebackstops are concave.
 8. The device of claim 1, wherein the backstopsare convex.
 9. The device of claim 1, wherein the backstops are flat.10. The device of claim 1, wherein the top end plate, bottom end plateand at least two struts comprise a ductile material.
 11. The device ofclaim 1, wherein the top base plate and bottom base plate each have anopening defined therein configured to allow bone growth into an openspace defined by the device.
 12. The device of claim 1, wherein the topbearing surface bottom bearing surface each have a plurality of teethextending outwardly therefrom.
 13. A method of intervertebral bodyfusion and distraction comprising: inserting a distractibleintervertebral body fusion device into an intervertebral disc spacedefined between adjacent vertebrae, the device including a top baseplate having a top bearing surface configured to interface with an endplate of a superior vertebra of the intervertebral disc space, a bottombase plate having a bottom bearing surface configured to interface withan end plate of an inferior vertebra of the intervertebral disc space,and at least two struts connecting the top end plate and the bottom endplate, each strut comprising first and second structural members and ablock, wherein each structural member is connected at one end to one ofthe end plates with a flexure member and at an opposed end to the blockwith a flexure member; distracting the device into an expandedconfiguration by actuating an expansion tool in a first direction todrive the blocks closer together, causing the struts to expand, therebydistracting the top end plate and bottom end plate with respect to eachother to expand an opening within the disc space.
 14. The method ofclaim 13, wherein the step of distracting the device into an expandedconfiguration causes the flexures connecting each structural member toone of the end plates to make rolling contact with a backstop providedby each end plate that guides a curvature of the flexure as the top andbottom end plates distract.
 15. The method of claim 13, furthercomprising: inserting a bone growth promoting material into the discspace into an open space defined by the device, the open space includingan opening extending through the top base plate and an opening extendingthrough the bottom base plate; allowing the bone growth promotingmaterial to aid in intervertebral fusion of the adjacent vertebrae whilethe device stably supports the vertebrae.
 16. The method of claim 13,wherein the device includes at least two struts on a first side of thedevice and at least two struts on a second side of the device, andwherein the step of distracting the device into an expandedconfiguration includes expanding the struts on each side of the device asame distance.
 17. The method of claim 13, wherein the device includesat least two struts on a first side of the device and at least twostruts on a second side of the device, and wherein the step ofdistracting the device into an expanded configuration includes expandingthe struts on each side of the device different distances.
 18. A methodcomprising: providing a distractible intervertebral body fusion device,the device including a top base plate having a top bearing surfaceconfigured to interface with an end plate of a superior vertebra of anintervertebral disc space and a bottom base plate having a bottombearing surface configured to interface with an end plate of an inferiorvertebra of the intervertebral disc space and at least two strutsconnecting the top end plate and the bottom end plate, each strutcomprising first and second structural members and a block, wherein eachstructural member is connected at one end to one of the end plates witha flexure member and at an opposed end to the block with a flexuremember; and providing instructions for performing an intervertebral bodyfusion and distraction procedure with the distractible intervertebralbody fusion device, the instructions comprising: inserting thedistractible intervertebral body fusion device into an intervertebraldisc space of a patient defined between adjacent vertebrae such that thetop bearing surface interfaces with an end plate of a superior vertebraof the intervertebral disc space and the bottom bearing surfaceinterfaces with an end plate of an inferior vertebra of theintervertebral disc space; distracting the device into an expandedconfiguration by actuating an expansion tool in a first direction todrive the blocks closer together, causing the struts to expand, therebydistracting the top end plate and bottom end plate with respect to eachother to expand an opening within the disc space.
 19. The method ofclaim 18, wherein the step of distracting the device into an expandedconfiguration causes the flexures connecting each structural member toone of the end plates to make rolling contact with a backstop providedby each end plate that guides a curvature of the flexure as the top andbottom end plates distract.
 20. The method of claim 18, wherein theinstructions further comprise: inserting a bone growth promotingmaterial into the disc space into an open space defined by the device,the open space including an opening extending through the top base plateand an opening extending through the bottom base plate; allowing thebone growth promoting material to aid in intervertebral fusion of theadjacent vertebrae while the device stably supports the vertebrae withzero, or fewer, degrees of freedom.
 21. A medical device capable ofbeing distracted from a collapsed configuration to an expandedconfiguration, comprising: a top end plate; a bottom end plate; and atleast two struts connecting the top end plate and the bottom end plate,each strut comprising first and second structural members and a block,wherein each structural member is connected at one end to one of the endplates with a flexure member and at an opposed end to the block with aflexure member, and wherein the blocks include apertures enablingoperable access to an expansion tool such that actuation of theexpansion tool in a first direction drives the blocks closer together,causing the struts to expand, thereby distracting the top end plate andthe bottom end plate with respect to each other.
 22. The device of claim21, wherein the top end plate, bottom end plate and at least two strutscomprise a one piece unitary body.
 23. The device of claim 21, whereinone of the end plates provides a backstop for each flexure connectingone of the structural members with one of the end plates, the backstopconfigured to provide a rolling contact with the flexure and guide acurvature of the flexure as the top and bottom end plates distract.